The invention relates to a control circuit of the preamble of claim 1 for a semiconductor component, particularly to a control circuit for an IGB transistor, to enable faster turn-on and turn-off of the transistor.
High-power IGB transistors typically have a gate charge of several microcoulombs and therefore in order to change transistor state quickly and with low losses, a current of several amperes has to be applied to control the gate. The gate voltage of an IGB transistor in a conducting state is typically of the order of +15 volts; if the voltage is too low, it may increase conduction losses in the component, whereas a gate voltage that is too high may destroy the component in case of a short-circuit. In a non-conducting state the gate voltage should achieve a negative value of at least 5 to 7 volts to prevent the capacitance between the collector and the gate from affecting the state of the gate in case of a rapid change in the collector voltage.
In change-over switches IGB transistors are usually in pairs and therefore to avoid a breakthrough situation during which the amount of current passing through the components might reach a full short-circuit current, the transistor that is currently on must be turned off before the opposite transistor is turned on. For this reason delays in power components should be as short and stable as possible. This can be achieved by providing the circuit with delay circuits that add what is known as dead time to the gate control of opposite components.
Accelerated gate control typically enables to achieve a decrease of as high as 20% in switching losses, which accounts for 10% of total losses.
The switching losses of IGB transistors depend on how rapidly a gate charge providing full conductivity can be fed to or removed from the component gate. The Miller plateau of an IGB transistor is typically at 10 to 11 volts and therefore the output voltage of the gate driver must be sufficiently high to allow current to be generated through the gate resistor.
It is easy to imagine that with an auxiliary voltage at +15 volts and the Miller plateau at 11 volts, an IGB transistor gate cannot be fed with a particularly high current. In a stationary state an increase in the auxiliary voltage alone may cause the risk of a component breakdown in case of short-circuit.
Although inexpensive gate driver microcircuits provided with a buffered output stage and galvanic separation implemented by means of optoelectronics are available, these circuits do not have a sufficient current supply capacity to control high-power IGB transistors. In addition, they usually have an output stage provided by means of bipolar technology, in which voltage loss in the direction of turn-on may be of the order of 2.5 volts. In other words, the difference to the voltage level of the Miller plateau of the IGB transistor is only 1.5 to 2.5 volts and thus rapid transfer of gate resistance is not possible.
To reduce voltage loss and to increase current supply capacity, the output of the circuits may be provided with a separate power stage, which is usually implemented by means of a p-type MOSFET connected from its source terminal to the positive supply voltage and by means of an n-type MOSFET correspondingly connected from its source terminal to the negative supply voltage. However, a problem that arises is that the gates of the buffer stage FET should be biased so that breakthrough cannot occur. This easily leads to a complicated and expensive end result.
Optically separated control circuits may have a transfer delay of several nanoseconds, and it may depend on the temperature, for example. For this reason dead time must be increased in the IGB transistors to be controlled in order to obtain a sufficient margin of certainty. On the other hand, using an extremely high-speed transformer connection for signal transfer usually increases costs and further complicates the circuit configuration.
Although gate drivers that remove the above problems are commercially available, they are usually very expensive and therefore not applicable to bulk products, such as frequency converters.